Atrial fibrillation (AF) is the commonest rhythm disturbance of the heart, and is a major cause of serious morbidity such as congestive heart failure and cerebrovascular embolism (`stroke'). More recently, AF has been found to be associated with significantly increased mortality, compared to patients in normal sinus rhythm. Importantly, the incidence of this arrhythmia increases with age, with the result that AF is fast becoming the latest `epidemic'in an aging population. The diagnosis and management of AF have therefore become an important and challenging aspect of cardiovascular medicine. However, progress in effectively treating AF has been slow, in large part due to a poor understanding of the underlying mechanisms of this arrhythmia. In this regard, recent studies indicate an important role for the pulmonary veins and the posterior left atrium (PLA) in the genesis of this arrhythmia. Several pioneering ablative procedures have therefore been performed in the PLA, albeit with mixed success. In the heart, G protein coupled receptors (GPCRs) and their cognate signaling partners, the heterotrimeric G-proteins, regulate most mechanical and electrical functions. The autonomic nervous system regulates critical cardiac parameters such as excitability, heart rate, force of contraction, conduction velocity and refractoriness. Activation of 2-adrenergic receptors, which are coupled to Gs, leads to an increase in conduction velocity and several other excitatory responses in the heart. Activation of muscarinic M2 receptors, which are coupled to Gi, leads to a marked shortening of refractoriness in the atria. In combination, these two limbs of the autonomic nervous system have been demonstrated to create substrate for AF. Thus, the adrenergic and muscarinic receptors or their partners Gi and Gs may be viable alternative targets for therapeutic strategies designed to modulate arrhythmogenic influences in the heart. The PLA may be an especially attractive target for these strategies, on account of a very robust and unique autonomic profile that is thought to be conducive to AF. Peptides which mimic the C-terminus of the various G1 proteins have been shown in a number of systems to selectively inhibit receptor signaling by serving as competitive inhibitors and blocking the ability of the receptor to couple to the appropriate G protein. In an attempt to modify substrate for AF. We propose to use peptides directed at the GPCR/G protein interface to selectively inhibit parasympathetic or sympathetic pathways in the PLA. We will use either the peptide alone (i.e. cell-penetrating peptides) or a minigene (plasmid) that can express the peptide. We will perform these studies in a) isolated canine cardiomyocytes, to study the effects of the peptide(s) on ion-channels that mediate autonomic effects, and in b) intact dogs, with localized injection into the PLA in order to inhibit vagally or adrenergically-mediated AF. The proposed studies are an important stride towards identifying novel therapeutics that may eventually be applied to the treatment of life threatening arrhythmias. Atrial fibrillation (AF) is the commonest rhythm disturbance of the heart, and is a major cause of serious morbidity such as congestive heart failure and stroke. However, currently available treatment options for AF are not very effective. We propose to use a new method to treat AF, by inhibiting the function of the nerves that are critical to the genesis of this arrhythmia. By using a new inhibitory protein, we will selectively block these nerves in the left atrium.